A British soldier from who went missing in southern Afghanistan has been found dead, the Ministry of Defence has confirmed.

The soldier left his base in central Helmand in the early hours of Monday morning and failed to return

It is believed the soldier was finishing sentry duty at a vehicle checkpoint Salaang and rather than join his colleagues appears to have left the small base and disappeared.

Military sources in the UK dismissed reports from Afghanistan that the soldier had been taken in an ambush, or that there had been a fire fight between insurgents and British forces.

A source in Whitehall said: "He had been on 'STAG' (sentry duty) at a small vehicle checkpoint and had just finished his shift. And then, for whatever reason, he walked off. Instead of going back into the base where the other soldiers were, he headed out of the base - and that was the last that was seen of him.

"We are still trying to work out exactly what has happened. There are always claims of responsibility for incidents, but we can only work with what we know. And all we know is that he has gone. At the moment, we cannot even be absolutely sure he is in the hands of the Taliban. There are criminal gangs operating in this area too.

The defence secretary, Liam Fox, told the Commons the man's next of kin have been contacted and will be kept informed.

The Taliban have made a number of claims related to the man's disappearance.

A Taliban spokesman told Reuters it had captured and killed a foreign soldier on Sunday after a firefight in the Babaji area of southern Helmand. In a separate claim to the Guardian, a Taliban spokesman said its fighters had killed the soldier in an ambush on Sunday afternoon, along with four to six other international troops.

Neither claim could be verified and the Taliban often make exaggerated claims for propaganda purposes.

The prime minister's spokesman said David Cameron was aware of the incident, but it would have no effect on a statement he intends to make to the Commons on Wednesday announcing that 500 to 800 extra British troops are to withdraw from Afghanistan.

Three members of Nato forces are known to have been previously captured in Afghanistan.

Sergeant Bowe Bergdahl, 25, from Hailey, Idaho, was taken prisoner in June 2009 in Paktika province in eastern Afghanistan and is thought to be in Pakistan.

In July 2010, two sailors from the US Navy went missing in Logar province, south of Kabul, prompting an extensive manhunt. The dead bodies of both sailors were found days later in Logar.

The Nato command in Afghanistan said the soldier had been listed as "duty status whereabouts unknown" and a search was under way.